SMA HAD NMAC WITH CESSNA IN TFC PATTERN CLASS D AIRSPACE. EVASIVE ACTION TAKEN ACFT GAR. SEE AND AVOID CONCEPT.

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AT THE END OF AN IFR FLT; AFTER DECLARING RWY IN SIGHT; I SWITCHED FROM BOSTON APCH ATC TO BEDFORD TWR; 4 MI OUT. I NEXT RPTED MID-FIELD R DOWNWIND AND WAS TOLD THAT I WAS '#3 FOLLOWING THE CESSNA ON R BASE.' A BARON WAS APCHING BUT NOT OVER THE RWY THRESHOLD. IMMEDIATELY AFTER I WAS GIVEN MY SEQUENCE; THE BARON REQUESTED CONFIRMATION OF CLRNC TO LAND. I; THEREFORE; COUNTED THIS BARON AS #1 IN THE SEQUENCE AND STARTED LOOKING FOR THE CESSNA ON R BASE. AS I GOT ANOTHER 1.5 MI DOWNWIND; I SAW A CESSNA ON LONG FINAL AND ASSUMED THAT THIS WAS THE #2 PLANE THAT I WAS FOLLOWING AND INFORMED TWR 'CESSNA IN SIGHT.' AS I GOT CLOSE TO LNDG; SUDDENLY ANOTHER CESSNA APPEARED BELOW ME AND I WENT AROUND. I INFORMED THE TWR OF THIS AND COMPLAINED THAT THEY HAD MISLED ME ON MY SEQUENCE BEING #3. THEY INFORMED ME THAT THEY DON'T COUNT AIRPLANES (IE; THE BARON) ON SHORT APCH (AND; IN THIS CASE; NOT YET OVER THE RWY). I BELIEVE THAT MOST PLTS ASSUME THAT ANY PLANE NOT ACTUALLY ON THE RWY IS COUNTED IN THE SEQUENCE. CTLRS SEEM TO THINK THEY HAVE SOME DISCRETION ON THEIR PART AS TO WHAT TO COUNT. OBVIOUSLY; AMBIGUITY; AS TO WHICH ACFT ARE COUNTED AND WHICH ARE NOT; CAN BE FATAL.